Acquiring the quantitative state-of-charge (SOC) distribution of a lithium-ion battery cathode is important to improve the performance of lithium-ion batteries for the application to large-scale devices. The conventional method to obtain SOC distribution, i.e., X-ray absorption spectroscopy (XAS), requires a synchrotron radiation facility, references that are difficult to prepare, and an investigation of the XAS spectra of the references. Herein, we demonstrate that laser-induced breakdown spectroscopy (LIBS) can acquire a quantitative SOC of a charged lithium-ion battery cathode with high accuracy by comparing the SOC values of charged cathodes containing Li₂NaV₂(PO₄)₃ as an active material determined by LIBS, XAS, and a charge–discharge curve. We perform LIBS measurements in a reduced argon atmosphere of 1000 Pa and use a calibration curve obtained by measuring the emission intensities at 610.4 nm of the pristine cathode. The SOCs of the charged cathodes agree well with the average SOCs estimated from the charge–discharge curves, whereas those determined by XAS do not. Therefore, LIBS is more suitable to acquire a quantitative SOC distribution of a lithium-ion battery cathode than XAS because LIBS is a laboratory-scale measurement and requires a single simple reference in addition to its high accuracy.